The present invention relates to circuits that can operate at low supply voltages. More particularly, the present invention provides circuits that function with supply voltages near or lower than the threshold voltage of transistors used in the circuit.
For some time, a trend in semiconductor circuits has been to decrease the feature size of transistors and to place more transistors in the same size area of silicon. This trend of decreasing feature size promises to bring the features of transistors into the nano-scale range so that the intrinsic speed of the devices, and thus the signal processing capability, can keep increasing. With continually shrinking feature size, supply voltages for circuits have also experienced a downward trend in order to maintain reliability, to reduce power density, and to avoid thermal problems. This downward trend in power supply voltages is beneficial for digital circuits because it reduces power consumption quadratically. However, the threshold voltages at which transistors operate have not decreased proportionally to the supply voltages because the transistors' threshold voltages cannot be reduced as aggressively. This is the case because lower threshold values create static leakage (i.e., unwanted electron/hole flow when a device is off) that would result in unacceptable circuit operation. Therefore, it is desirable to provide analog circuits that can operate with supply voltages near or lower than the threshold voltage of the transistors in those circuits.